Ink jet printers are well known in the art. Generally, an ink jet printer includes an array of nozzles or orifices, a supply of ink, a plurality of thin channels connecting the array of nozzles with the ink supply, respectively, a plurality of ejection elements (typically either expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the array of nozzles, respectively, and suitable driver electronics for controlling the ejection elements. Typically, the array of nozzles and the ejection elements along with their associated components are referred to as a print head. It is the activation of the ejection elements that causes drops of ink to be expelled from the nozzles. The ink ejected in this manner forms drops which travel along a flight path until they reach a print medium such as a sheet of paper, overhead transparency, envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. Typically, the ejection elements are selectively activated or energized as relative movement is provided between the print head and the print medium so that a predetermined or desired print image is achieved via the collective effect of the placement of the drops.
Generally, the array of nozzles, supply of ink, plurality of ejection elements and driver electronics are packaged into an ink jet cartridge. In turn, the printer includes a carriage assembly for detachably mounting the ink jet cartridge thereto. In this manner, a fresh ink jet cartridge may be installed when the ink supply of the current ink cartridge has been consumed. In other ink jet printers, the ink supply is remotely located from the print head and ink is delivered to the print head via a supply tube.
To keep an ink jet printer in proper working order, a variety of maintenance actions, such as capping, wiping, normal flushing, power flushing, normal purging and power purging, have been developed. Most of these maintenance actions are directed toward preventing the array of nozzles from becoming clogged with stale ink or other debris. When not in use, the print head is sealed off from ambient air by a cap. In this manner, the evaporation rate of any solvents or other volatiles contained within the ink is reduced and the ink is less prone to clumping. A wiper blade is typically employed to squeegee any excess ink or other debris off from the face plate of the array of nozzles. This cleaning action is typically performed both prior to capping and prior to printing. A normal flush involves firing each nozzle in the array of nozzles a predetermined number or times to expel ink that may be beginning to clump. A power flush is similar to a normal flush except that the number of time each nozzle is fired is substantially greater than that for a normal flush. A normal purge involves applying a vacuum for a predetermined amount of time to the array of nozzles to suck out ink. A power purge is similar to a normal purge except that the amount of time that the vacuum is applied is substantially greater than that for a normal purge.
Since ink that is consumed during maintenance actions is not available for printing, it is desirable to keep the maintenance actions to a minimum. In this manner, the overall cost to the user will be reduced by providing greater ink utilization. On the other hand, it is desirable to keep the print head operating at optimum conditions so that a high degree of reliability and print quality is achieved. In this manner, the user does not have to waster paper, ink and time reprinting items that were printed improperly. Thus, a tension exists between minimizing maintenance actions to conserve ink and providing a print head that is ready to produce high quality printed images on demand.
Recently, the postage meter industry and other envelope printing industries have begun to incorporate ink jet printers. A typical postage meter (one example of a postage printing apparatus) applies evidence of postage, commonly referred to as a postal indicia, to an envelope or other mailpiece and accounts for the value of the postage dispensed. As is well known, postage meters include an ascending register, that stores a running total of all postage dispensed by the meter, and a descending register, that holds the remaining amount of postage credited to the meter and that is reduced by the amount of postage dispensed during a transaction. Because U.S. Postal Service regulations require that postage be paid in advance, it had traditionally been required that the user of a postage meter periodically present the meter to a Postal Service employee for recharging. However, more recently it is possible to recharge a meter remotely using telephone communications. At the time of recharging, the user paid to the Postal Service the amount of postage to be credited to the meter and the meter is recharged by increasing the setting of the descending register by the amount paid. The postage meter generally also includes a control sum register which provides a check upon the descending and ascending registers. The control sum register has a running account of the total funds being added into the meter. The control sum register must always correspond with the summed readings of the ascending and descending registers. The control sum register is the total amount of postage ever put into the machine and it is alterable only when adding funds to the meter. In this manner, the dispensing of postal funds may be accurately tracked and recorded.
Due to the inherent nature of printing an indicia of value (a postal indicia being the equivalent of money), several issues arise with utilizing ink jet printing in a postage printing device. For example, if a general purpose ink jet printer runs out of ink while printing a document or suffers poor print quality, then the user merely installs a new cartridge and reprints the document. Although supplies are not optimally used, no direct loss of money occurs. On the other hand, if a postage printing device runs out of ink or suffers poor print quality while printing a postal indicia, then the user loses money because the postal funds associated with that postal indicia cannot be recovered. Therefore, it is highly desirable to avoid running out of ink and ensuring quality printing in a postage printing system such as a postage meter.
Thus, there is a need in ink jet printers to balance the competing interests of optimizing ink usage and maintaining print head readiness and print quality. In this way, the actual cost of the ink may be reduced because less ink is consumed during maintenance actions. However, this situation is often complicated due to the wide range of usage patterns that exist among users.